英语听力：探索发现 2014-06-27 BBC 地平线：陨石的真相-20

Oh, perfect, perfect, perfect. Now were seeing the fireball come in, its brighter than the Sun and then, Kapow, it hits the surface. Jeez! This whole region, downrange, would have been incinerated. It would have been incinerated just by this plasma,

英语听力：探索发现 2014-06-25 BBC 地平线：陨石的真相-19

So this was serial number one. It was built during the Apollo time. I guess because they thought there would be several of them made, but this is the first one and the last one and is the only one like it in the world. This is NASAs Vertical Gun Rang

英语听力：探索发现 2014-06-22 BBC 地平线：陨石的真相-18

Down on the ground, the scale of impact is even more breathtaking.The crater is more than 1000 meters across and nearly 200 meters deep.The forces here were enoumous.The impact on this solid rock into this pulverized mochi,just bustled in your hand.Y

英语听力：探索发现 2014-06-20 BBC 地平线：陨石的真相-17

Really, its a balance between the size of the object, its speed into the atmosphere and critically the altitude at which it explodes. Too high, if its too small and it explodes too high, the shockwave has little effect on the ground. If its quite low

英语听力：探索发现 2014-06-18 BBC 地平线：陨石的真相-16

Most of the damage from an explosion like this is actually the blast wave. Its the very high winds. Mark created a simulation to see what size an asteroid would need to be to be able to generate such destructive power. In this simulation I include mo

英语听力：探索发现 2014-06-15 BBC 地平线：陨石的真相-15

On June 30th, 1908, a huge explosion tore through the forest of Tunguska, Siberia. It was 20 years before the Russians mounted an expedition to the site. What they found astonished them. 60 million trees across an area the size of London had been lev

英语听力：探索发现 2014-06-13 BBC 地平线：陨石的真相-14

So was this a near miss for us? If the asteroid had been in a different part of its orbit, so it didnt hit this year but it hit next year, it would have still hit us on February 15th. But instead of coming in over Russia, it would have come in over t

英语听力：探索发现 2014-06-11 BBC 地平线：陨石的真相-13

There it is! Yep. Its about 90 kilometres up, at that stage, travelling at 17.5 kilometres per second. Using the different camera positions, scientists have pinpointed the exact position at which the meteor entered the atmosphere. And by tracking the

英语听力：探索发现 2014-06-29 BBC 地平线：陨石的真相-21

Comparing the effects of an airburst with a ground strike, it seems the Chelyabinsk got away lightly. Its estimated that the largest piece to hit the ground weighed 500 kilos, a fraction of the asteroids original mass of 7,000 tonnes. Now if a piece

英语听力：探索发现 2014-07-16 BBC 地平线：陨石的真相-28

An asteroid strike would create a huge explosion. NASA feared this might even be mistaken for a nuclear bomb. We wanted folks to know this was a natural event by Mother Nature rather than some sort of man-made event like a missile or something dreadf

英语听力：探索发现 2014-07-18 BBC 地平线：陨石的真相-29

Approaching from beneath the planet, asteroid 2012 DA14 passed inside the orbit of our geostationary satellites before heading off to the north. This asteroid had been successfully tracked for a year. And despite its proximity, scientists knew that i

英语听力：探索发现 2014-07-20 BBC 地平线：陨石的真相-30

It came in in the daytime sky out of the Sun. Weve got telescopes looking out there for these objects, but they only work at night. Radar doesnt help either because to really use radar to find these objects above the atmosphere, you have to know exac

英语听力：探索发现 2014-07-23 BBC 地平线：陨石的真相-31

When you consider the Earths history stretching over billions of years, its clear that meteorite impacts, far from being unexpected, are just a normal part of the life cycle of our planet. But that is not how they seem to us. The Chebarkul meteorite

英语听力：探索发现 科学新发现：我们的大气层-11

The planet also began releasing oxygen, thanks to an ancient adoption of photosynthesis. A very ancestral primitive plant cell either engulfed or tried to digest or surrounded a cyanobaterium and instead of digesting it, it actually became an organ w

英语听力：探索发现 科学新发现：我们的大气层-1

In the empty blackness of space surrounded by hostile planets and freezing vacuum, planet Earth is an oasis of life. And all because of our protective cocoon of gas. This blanket of air shapes everything we see on Earth. It protects, insulates and su

英语听力：探索发现 科学新发现：我们的大气层-2

Our chemistry, our very nature, our metabolism is completely shaped by the atmosphere. To appreciate our atmosphere, well rise through its layers, travelling from the planets surface, miles up above mountains and the clouds, to the very edge of space

英语听力：探索发现 科学新发现：我们的大气层-3

My crew chief said, are you ready? I said, Im ready, lets go. It was a very smooth take-off and I started ascending up, up, up. 30 minutes after take-off, Kittinger reaches 29,000 feet, the middle of the troposphere. At this altitude, the air is so t

英语听力：探索发现 科学新发现：我们的大气层-4

Kittinger is in the mid stratosphere, a record 102,800 feet high. And it is now the time to make the long, swift journey down. When it came time for me to jump, I was ready to jump because I was heading back toward a more friendly environment, a frie

英语听力：探索发现 科学新发现：我们的大气层-5

英语听力,探索发现,科学新发The mission had been a stunning success: Kittinger skydiving record stands to this day. He takes his first deep breath of air with a new found appreciation for our atmosphere. It was relief to be back on the plan

英语听力：探索发现 科学新发现：我们的大气层-6

In contrast, Jupiter and Saturn are all atmosphere. Unlike Earth, Jupiter doesnt really have a solid surface that you could stand on and then have your head up in the atmosphere looking at the clouds. Its just a giant, giant gas ball. Only one other